A MIMO wireless communication system, as shown in FIG. 1, is one which comprises a plurality of antennas 10 at the transmitter 11 and one or more antennas 12 at the receiver 13. The antennas 10, 12 are employed in a multipath rich environment such that due to the presence of various scattering objects (buildings, cars, hills, etc.) in the environment, each signal experiences multipath propagation. Thus a cloud shape 14 is shown in FIG. 1 to represent the scattered signals between the transmit and receive antennas. User data is transmitted from the transmit antennas using a space-time coding (STC) transmission method as is known in the art. The receive antennas 12 capture the transmitted signals and a signal processing technique is then applied as known in the art, to separate the transmitted signals and recover the user data.
MIMO wireless communication systems are advantageous in that they enable the capacity of the wireless link between the transmitter and receiver to be improved compared with previous systems in the respect that higher data rates can be obtained. The multipath rich environment enables multiple orthogonal channels to be generated between the transmitter and receiver. Data for a single user can then be transmitted over the air in parallel over those channels, simultaneously and using the same bandwidth. Consequently, higher spectral efficiencies are achieved than with non-MIMO systems. Channel SNIR (Signal to Noise and Interference Ratio) can also be improved via use of beamforming.
Lucent have proposed a MIMO technique called Per-Antenna Rate Control (PARC) which is described in the document ‘Increasing MIMO throughput with per-antenna rate control’, 3GPP TSG-R1#21(01)0879, 27-31 Aug. 2001, Turin, Italy. PARC differs from previous MIMO techniques such as code reuse and differential space-time transmit diversity, in that modulation and coding schemes (MCS) and hence data rates on each transmitter antenna may be different. Adaptive Modulation and Coding (AMC) is also used, in that the MCS can be changed whilst the communication link is active in order to react to changes in channel performance. A benefit of the PARC technique is that by introducing flexibility in the data rate on each antenna, the overall throughput of the system in increased. The decision regarding which MCS to use is made at the receiver on the basis of a performance evaluation on the downlink (received) signal, e.g. on the basis of the SNIR (Signal to Noise and Interference Ratio). It is therefore necessary for the receiver to signal to the transmitter via the uplink to request the chosen MCS. The receiver sends a corresponding identification number via a feedback sub-channel to the transmitter. Given that there are multiple antennas and multiple MCS, there are potentially a very large number of combinations which could be chosen. However, there is limited feedback capacity within the uplink and the larger the number of combinations, the larger the number of bits which are required on the uplink to indicate the chosen MCS combination (or configuration).
Lucent proposes, therefore, that a limited set of MCS combinations are used. The limited set of MCS combinations are chosen such that throughput is maximised for a given geometry and given maximum acceptable frame error rate (FER). Typically the number of MSC combinations is limited to 32 or fewer which means that they can be indexed by 5 bits per transmission time interval (TTI).
One of the problems with this approach is that certain MCS configurations which may be useful are not available as they are not included within the fixed set, even though the transmitter is physically capable of using them.
The invention seeks to provide a method for controlling a communications link which mitigates at least one of the problems of known methods.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.